1. Field of the Invention
The present invention relates to a method of modifying powder materials used for manufacture of machine parts by powder metallurgy, and more particularly relates to an iron-based powder mixture for powder metallurgy and a manufacturing method thereof, producing an improved powder mixture having stabilized apparent density, less segregation of additives, and superior fluidity characteristics when discharged from a hopper; which method requires less force when ejected from a die in a compacting process; and which material contains no zinc or small amounts of zinc at most.
2. Description of the Related Art
Conventional powder materials used for machine parts have been mixed powders in which the alloying powder of the components for the improvement of solid-state properties, such as copper, nickel, graphite, and phosphorus, was mixed into an iron powder. A lubricant such as zinc stearate was also mixed into the powder to reduce abrasion resistance during compressed molding. However, these powder mixtures tended to experience powder segregation, which readily occurred during transport after mixing, loading and unloading to and from a hopper, or during molding, because the powder mixture contained powders of different sizes, shapes, and densities.
This segregation caused fluctuations in product composition, which increased fluctuation of dimensional changes and strength, and thus produced defective products. Furthermore, graphite and the like, due to their properties as impalpable powders, enlarge the specific surface area of the powder mixture, thus impairing fluidity. This impairment lowers the injection speed to the die, which also reduces the production speed of the green compact. Technology for preventing segregation of these powder mixtures is disclosed in Japanese Patent Laid-Open No.56-136901 or No. 58-28231, in which a binder is used for preventing segregation. However, the more the amount of binder that is to improve segregation of the powder mixture, the lower the fluidity of the powder mixture.
A powder in which graphite was adhered to the surface of the iron-based powder with a binder of zinc stearate was disclosed in the Japanese Patent Laid-Open No. 1-219101. Also, we have proposed a method employing a metal soap and a fatty acid as a binder in Japanese Patent Laid-Open No. 3-162502. However, all of the above mentioned methods included zinc and other metallic elements in the binders, which caused a major problem since metallic elements in the binders, as oxides, contaminated the inside of the furnace, or varied the composition of the sintered body during sintering.
To overcome these problems, some methods employ binders having no metallic elements, as disclosed in Japanese Patent Publication No. 60-502158 and Japanese Patent Laid-Open No. 2-217403, wherein the binders themselves do not have a lubricating function, and thus zinc stearate was added as the lubricant in the end. Therefore, as described before, zinc in the lubricant contaminated the inside of the furnace as an oxide or varied the composition of the sintered body.